Oscillator circuits are used with digital electronic circuits (e.g., microcontrollers and microprocessors) for a variety of uses including to generate a clock signal (e.g., a frequency signal) for synchronizing, pacing and coordinating the operations of the digital electronic circuit. Some common forms of oscillator circuits are crystal oscillator circuits and voltage controlled oscillators (VCO) circuits.
Often, it is desirable to generate a frequency signal that is double the frequency of the signal generated by the oscillator circuit. Frequency doubler circuits are used to double the frequency signal generated by an oscillator circuit. A frequency doubler circuit is a low cost alternative to manufacturing a larger circuit to generate a larger frequency signal. Rather, a smaller, less expensive circuit is manufactured, and the frequency signal it generates is doubled.
FIG. 1 is a block diagram 100 of a frequency doubler circuit 100 in accordance with the conventional art. Oscillator circuit 105 generates a first frequency signal 110. Delay locked loop circuit 115 receives first frequency signal 110 and generates a second frequency signal 120 that is double the value of first frequency signal 110. For example, if the value of first frequency signal 110 is substantially 24 MHz, the value of second frequency signal 120 will be substantially 48 MHz.
Often, frequency doubler circuit 100 comprises control loop circuit 135 for controlling the duty cycle of frequency doubler circuit 100. Filter circuit 125 converts second frequency signal 120 into DC voltage 130. Control loop circuit 135 receives DC voltage 130 and generates output current 140 for controlling the biasing of internal components of frequency doubler circuit 100 (e.g., inverters and transistors).
Some frequency doubler circuits comprise a plurality of capacitors. A disadvantage to the use of capacitors is that capacitors often vary due to the fabrication process or to temperature. In current frequency doubler circuits, it is not always of generally possible to adjust the current charging the capacitors to account for process variation. In order to ensure that capacitors are to the proper specifications, all capacitors should be tested and those that are not to specification are discarded, resulting in wasted parts.
Furthermore, devices comprising a frequency doubler circuit often require a highly controlled duty cycle to perform certain applications. Conventional frequency doubler circuits do not provide a highly controlled duty cycle as they do not compensate for capacitor process variation.